The use of fiber reinforced plastic for various applications is well known. Due to their strength and lightweight, such compo sites are common in the aerospace, automotive, marine, and construction industries. Currently there is limited capacity for fiber reinforced plastics to conduct current in a direction orthogonal to the linear orientation of the primary fiber reinforcement.
Lack of sufficient conductivity, particularly in a through thickness (-z axis) direction (e.g., interplay direction) may result in large voltage potentials to build up within and on the surfaces of the fiber reinforced plastic. These voltages are relieved through energetic phenomena termed: “sparking,” “edge glow” or “edge emission”. This can be a significant concern, particularly in environments where fiber reinforced composites are used in flammable environments and where Federal regulations impose stringent standards on commercial aircraft. These regulations are directed toward reducing the possibility that fuel or fuel vapors in or around a fuel tank might be ignited by ignition sources such as electrical discharges or sparks. These discharges can be produced by the imposition of electrical potentials or currents, such as those generated by direct lightning strikes.
Lack of through thickness conduction of larger currents (i.e., current density) may result in the disassociation of the conductive carbon fibers from the matrix polymer during high current conduction due to excessive heating of the carbon fibers and surrounding matrix. In addition, the volumetric conduction of current may also induce the localized heating of composite surfaces, which can result in the ejection of particles. Particles that are near an edge of the fiber reinforced plastic articles may be of particular concern.
Standard practice could be to increase conductive filler content within the polymer matrix and on the surface of the layers of fiber reinforcement to reduce the voltage between the layers and to allow current pathways to exist in the orthogonal directions. A previous similar solution has been to incorporate metalized spheres within the polymer matrix. The metallization could be done using a conductive element, such as nickel. Such practices, however, have lacked uniformity of conduction throughout the composite structure toxicity of the metal, oxidation and sensitivity to moisture.
Accordingly, those skilled in the art continue with research and development efforts to overcome z-direction conductivity issues in the field of fiber reinforced plastics.